1. Field of the Invention
This invention relates to an apparatus such as a compact disk player, an optical disk apparatus or an optical card apparatus for scanning a recording medium with a condensed light beam while effecting auto focusing control and/or auto tracking control, thereby accomplishing recording and/or reproduction of information.
2. Description of the Related Art
In an information recording-reproducing apparatus for recording information on a recording medium and reading out the information recorded on the recording medium, auto focusing (hereinafter referred to as AF) control for condensing and controlling a recording-reproducing laser beam projected on the surface of the recording medium and auto tracking (hereinafter referred to as AT) control for position-controlling a condensed laser beam spot, to follow tracks formed on the surface of the recording medium have heretofore been effected.
On the other hand, in such AF control or AT control, the presence of an abnormality such as dust or flaws on the recording medium has sometimes caused the control operation to exceed a predetermined tolerance (so-called AF failure or AT failure). Therefore, there have been proposed various methods of preventing such a condition. The basic concept of such prior-art methods will hereinafter be described with reference to FIGS. 1 to 4 of the accompanying drawings. Here, a three-beam method and an astigmatism method will first be described as a popular AT and AF control method, and then the prior-art method of preventing AT failure will be described with AT control taken as an example.
FIG. 1 is a perspective view schematically showing the construction of an optical information recording-reproducing apparatus. FIG. 2 is a block diagram showing the construction of a photodetector shown in FIG. 1 and a circuit for producing an AF/AT control signal.
In FIG. 1, reference numeral 47 designates a semiconductor laser which is a light source, reference numeral 48 denotes a collimator lens, reference numeral 49 designates a light beam shaping prism, reference numeral 50 denotes a diffraction grating for dividing a light beam, reference numeral 40 designates a beam splitter, reference numeral 45 denotes a reflecting prism, reference numeral 46 designates an objective lens, reference numeral 41 denotes an astigmatism condensing lens system, and reference numerals 42-44 designate photodetectors.
A light beam emitted from the semiconductor laser 47 becomes a divergent light beam and enters the collimator lens 48, by which it is made into a parallel light beam. This parallel light beam is shaped into a predetermined light intensity distribution by the light beam shaping prism 49, whereafter it enters the diffraction grating 50, by which it is divided into three light beams (0-order diffracted light and .+-.1st-order diffracted lights). These three light beams enter and rectilinearly pass through the beam splitter 40, and are further reflected by the reflecting prism 45 and enter the objective lens 46, and pass therethrough, whereby they are condensed and form three light beam spots S1 (corresponding to +1st-order diffracted light), S2 (corresponding to 0-order diffracted light) and S3 (corresponding to -1st-order diffracted light) on a recording medium 51.
The light beam spots S1 and S3 are positioned on adjacent tracking tracks 52.sub.1 and 52.sub.2, respectively, formed on the recording medium 51, and the light beam spot S2 is positioned on an information track 53 between the tracking tracks. The medium 51 is driven in the direction of arrow X by a motor, not shown. Recording or reproduction of information is effected on the information track 53 by the spot S2.
The reflected lights from the light beam spots S1, S2 and S3 formed on the recording medium pass through the objective lens 46 and are thereby made into substantially parallel light beams, and are reflected by the reflecting prism 45. Of these reflected lights, the reflected light from the light beam spot S2 enters the four-division photodetector 43. Also, the reflected lights from the light beam spots S1 and S3 enter the photodetectors 42 and 44, respectively.
As shown in FIG. 2, an AF control system 61 is designed to add the photocurrent outputs I.sub.A, I.sub.B, I.sub.C and I.sub.D of the divided elements A, B, C and D, respectively, of the four-division photodetector 43 as indicated by (I.sub.A +I.sub.D) and (I.sub.B +I.sub.C), and to output the difference between the added values as an output V1 for AF control.
Also, an AT control system 62 is designed to output the difference between the photocurrent outputs of the photodetectors 42 and 44 as an output V2 for AT control.
An information reproducing system 60 is designed to output the sum total of the outputs of the divided elements A, B, C and D of the aforementioned four-division photodetector 43 as an output V3 for information reproduction.
That is, in the AF control system 61, when the light beam spot S2 is focused on the information track and forms the smallest spot thereon (i.e., during in-focus), it is projected as a circular spot onto the four-division photodetector 43, and substantially equal quantities of light enter the divided elements A, B, C and D and the output V1 for AF control becomes substantially zero. Also, during an out-of-focus state, the light beam spot S2 is projected as an elliptical spot onto the four-division photodetector 43 by the astigmatism condensing lens system 41, and the output V1 for AF control varies as shown in FIG. 3A. The horizontal axis of the graph of FIG. 3A represents the distance between the lens 46 and the recording medium.
On the other hand, in the AT control system 62, when the light beam spots S1 and S3 are uniformly positioned on the corresponding tracking tracks (that is, when the spot S2 is applied to right above the information track), substantially equal quantities of light enter the photodetectors 42 and 44 and the output V2 for AT control becomes substantially zero. Also, when the spot S2 deviates from the center of the information track, the signal V2 for AT control varies correspondingly to the difference between the quantities of reflected light from the spots S1 and S3, as shown in FIG. 3B. The horizontal axis of FIG. 3B represents the distance of the beam spot S2 from the center of the information track in a direction perpendicular to the track indicated by arrow Y.
The AF and AT control systems independently drive AF and AT actuators, respectively, not shown, so that the outputs V1 and V2 for control of the respective systems may become less than a predetermined allowable value or substantially zero, and control the position of the objective lens relative to the recording medium.
An example of the AT control and AF control systems in a state in which there is no impediment such as dust or a flaw on the recording medium has been described above.
A description will hereinafter be provided of a heretofore proposed system for preventing AF failure and AT failure in a state in which there is an impediment such as dust or a flaw on the recording medium.
FIG. 4 is a block diagram of a prior-art circuit for preventing AT failure. A control signal input from a terminal 63 is a signal which exhibits the same behavior as the aforedescribed signal V2 for AT control, and this signal for AT control (hereinafter referred to as the AT error signal) is input to an AT abnormality detecting circuit 64 and a sample hold circuit 65 for sampling and holding the AT error signal in accordance with the output signal of the AT abnormality detecting circuit 64.
Now, when there is no impediment which will cause AT failure, the absolute value of the AT error signal level is a predetermined value or less. The AT abnormality detecting circuit 64 detects by a window comparator, not shown, that the signal level is the predetermined value or less, and causes the sample hold circuit 65 to assume its sampling condition. On the other hand, when there is any impediment which will cause AT failure, the absolute value of the AT error signal level exceeds the predetermined value, and the AT abnormality detecting circuit 64 detects the abnormality of AT by the window comparator, not shown, and opens the switch of the sample hold circuit 65 to cause this circuit to assume its holding condition. This holding condition is released after the lapse of a predetermined time or at a point of time at which the aforementioned impediment has passed, and the sample hold circuit restores its sampling condition.
Also, in the AF failure preventing method, use is made of a circuit of the same construction as that of FIG. 4, and the aforementioned signal V1 for AF control is input from the terminal 63. This signal for AF control (hereinafter referred to as the AF error signal) is input to the AF abnormality detecting circuit 64 and the sample hold circuit 65 for sampling and holding the AF error signal in accordance with the output signal of the AF abnormality circuit 64, and the same operation as that in the AT failure preventing system is performed.
In the prior-art system, by the above-described operation, any impediment is detected, and only during a time in which there is the impediment, a control signal before the occurrence of the impediment is output in a false fashion, thereby minimizing the influence imparted to the control system by the impediment.
However, in the prior art described above, the AF error signal and/or the AT error signal is held at a point in time at which an impediment such as dust or a flaw on the recording medium has been detected and thus, the AF and/or AT actuator is controlled by a signal which has already exceeded an allowable predetermined value, and this has led to the problem that accurate control cannot be accomplished.